You're a scientist, and your invention, a pulsating, glowing orb called the Sentinel, absorbs and stores energy from a variety of sources. Now Earth is threatened by deadly aliens and you must use your creation to save the planet. The Sentinel absorbs energy from every power pod or energy capsule you shoot with your Atari light gun. When an energy source is hit, a beam blasts out from the Sentinel, strikes the target with hissing force, and draws energy back into your Sentinel. It's up to you to protect the Sentinel as it glides over the planets.